1. Field of the Invention
The present invention relates to a method of reproducing a recording medium and an apparatus for reproducing the same. More particularly, the present invention relates to a method of reproducing a recording medium by compressing moving picture information, and an apparatus therefor.
2. Background of the Invention
There has been known one of the CD-ROM (CD-Read Only Memory) standards, which is called a video-CD. The video-CD in which audio information as well as moving picture information has been recorded has been put to practical use as a recording medium for a so-called "Karaoke"
In the video-CD, a moving picture signal as well as an audio signal has been compressed and encoded in conformity with the MPEG (Moving Picture Coding Expert Group) standards before being recorded.
In other words, the audio signal and moving picture signal are recorded in the following manner. The audio signal is converted into a digital signal, and then compressed by the compression coding system utilizing an audition psychology characteristic before being recorded. The moving picture signal is converted into a digital signal, and one screen is blocked in a plurality of blocks. Each of the blocks is subjected to discrete cosine transformation (DCT) so that the coding quantity is decreased in accordance with the degree of a change of an image. Codes of short lengths are allocated to the codes which have been subjected to the DCT in accordance with a VLC (Variable Length Code), as they are higher in the frequency of occurrence, so that the coding quantity are reduced as a whole.
In the case of moving picture data, a difference between an actual screen and a screen a change of which is predicted from a past screen and future screen is extracted. Then, the difference thus extracted and the predicted quantity of change are recorded, and the data quantity is also compressed. However, since a demodulated picture cannot be obtained from only data for the difference and the predicted quantity of change, picture data for one screen (1 frame) which is a reference to the data for the difference and the predicted quantity of change is compressed and encoded as it is, without referring to other pictures. In this manner, the reference picture thus compressed and encoded without referring to other pictures is called an "I-picture" (Intra Picture). Other pictures which contain change information for the I-picture is called a "P-picture" (Predicted Picture) or a "B-picture" (Bi-directional Prediction Picture).
The audio signal and moving picture signal are recorded, as shown in FIG. 1, on a disc by a sector unit established by the CD-ROM standards. In other words, in FIG. 1, Symbol V designates a sector (hereinafter referred to a "video sector") of moving picture data which has been compressed and encoded, and A designates a sector (hereinafter referred to an "audio sector") of audio data which has been compressed and encoded. Since the audio data is less than the moving picture data in the quantity of data, it is recorded in the ratio of one audio sector to a plurality of video sectors, as shown in FIG. 1.
As will be described in more detail later, information on a distinction between the audio sector and the video sector, information on an absolute time on a disc, and information on a time when a moving picture or sound is outputted at the time of reproducing (information on a correspondence in a reproducing timing between the moving picture and sound) are recorded on the respective sectors. The reproducing apparatus reproduces each music and a moving picture associated with that music on the basis of those information.
As is apparent from the above description of the compression encoding system for moving picture data, an accurate demodulated picture cannot be obtained without the I-picture. For this reason, when reproduction is started from the halfway position of a certain track (one track corresponds to one music; hereinafter, a sequence of events such as one music will be called one (1) sequence), it is difficult to obtain a accurate reproduced image unless the position of the I-picture is known.
In the format of the video-CD, in order to maintain a given image quality, the I-pictures are arranged in such a manner that one I-picture is necessarily recorded within 2 seconds. However, in the format of the original video-CD (hereinafter referred to as an "old video-CD format"), no information on the position of the I-picture is recorded on a disc. For this reason, the old video-CD format did not facilitate the start of reproduction from the halfway position of a certain track.
Thereafter, in the format of a revised video-CD format (hereinafter, referred to as a "new video-CD format"), a halfway position on a track where an image can be reproduced (hereinafter referred to as an "entry point"), after being converted into time information, is recorded on an area (a first track 1 as will be described later) where information on the number of music on the disc and recording data on the disc is recorded. The entry point is directed to information on the position of the I-picture and enables a picture to be reproduced even at the time of partially reproducing or forward feed reproducing by reproducing while referring to that point.
In the video-CD, data which has been subjected to a compression processing is recorded as the moving picture signal.
That data contains I-picture data which enables one screen to be reproduced by decoding the compression-processed data independently because the data has been subjected to compressing and encoding within one screen as described above, P-picture data which cannot be decoded without use of the I-picture, and B-picture data which cannot be decoded without use of the I-picture data and the P-picture data.
In the normal reproduction, those I-, P- and B-pictures are decoded, respectively, to output the reproduction of the moving picture. In the high-speed reproduction such as FF (fast forward) or REW (rewind), the I-picture data may be extracted discretely from the disc and then displayed.
In the high-speed reproduction, as the practical operation, the I-picture data is searched through the reproducing operation, and then decoded so that the decoded data is continuously outputted to be displayed as it looks like a still picture. Thereafter, a track jump is executed so that an I-picture is searched by executing the reproducing operation again from a jump completion point. Then, after the decoding processing is completed, the present screen is switched to a previous screen so that a picture of the I-picture decoded at this time is outputted to be displayed, and a track jump is executed. The above operation is repeated.
However, in the case where no I-picture could be found at the time of the reproducing operation, the picture of the previous I-picture is caused to be displayed for a long period of time.
For example, because the I-picture cannot be found in the duration where an optical head is scanning between two music on the disc, a picture which has previously been decoded and outputted to be displayed is continuously being displayed. Therefore, the operation is not changed to the high-speed reproducing operation.
Also, if the I-picture is decoded and display is switched immediately when the I-picture has been found, the duration from the start of displaying a certain picture to the start of displaying a next picture, that is, the length of a period of time during which each picture is displayed is dispersed. The shortened or lengthened period of time for displaying each picture makes its visibility very poor as a high-speed reproduced picture.
The above difficulties result in such problems that the visibility of a search screen as well as the usability are lowered.
Moreover, a Karaoke system has been realized which uses a video-CD conforming to the old video-CD format as a recording medium. However, in Japan, in recent years, music programs are reduced from TV programs whereby the entire music is difficult to memorize. Also, the Karaoke houses in which Karaoke systems are installed are increased. The account of loyalty to copyrights is shifted from a music unit system to a time charge system. Thus, the environments surrounding a Karaoke market are changed as a result of which the market has made the following demands for the Karaoke systems.
That is,
1) A user wishes to sing only the characteristic portions of music which is a so-called "impressive" ("Sabi" in Japanese). PA1 2) The user wishes to sing only the first or second part of a poem or to finish with singing of the second part. PA1 3) The user wishes to forwardly feed parts of music irrelevant to singing such as an interlude or a weak phrase for omission.
However, in the reproducing apparatus having the video-CD as a recording medium such as the conventional Karaoke system, there is no means for knowing the position of a specified partial sequence. For this reason, the user is required to search the start and end points of the partial sequence to manipulate the reproducing apparatus so as to realize the specified partial reproduction or omission described in the above items 1) to 3). This is troublesome. In particular, in the case of limiting the position of the partial sequence depending on music, for example, in the case of reproducing only the portion of "Sabi" (impressive), the operator must manipulate the reproducing apparatus at not only the start position of the partial sequence but also the end position of the partial sequence. Thus, the manipulation is troublesome.
As described above, in the old video-CD format, information on the position of the I-picture is not recorded on a disc. For this reason, when starting reproduction from the middle of certain music as in the above items 1) to 3), a moving picture associated with such a situation is difficult to reproduce.
In the new video-CD format, since the position of the I-picture is known as an entry point, the moving picture can be reproduced even from the middle of music. However, in the case of starting reproduction from a specified portion of music as in the above items 1) to 3), it can be specified from which entry point reproduction should be preferably started.